This invention concerns asphalt pavements. It also concerns systematic procedure for mix design, mix analysis, and job control for producing a uniform asphalt pavement of substantial length having uniform physical properties.
In the plant-mix type pavement construction, a road bed and/or subgrade on which an asphaltic concrete mixture is laid generally are prepared by grading, compacting, and leveling. The asphaltic mixture is prepared at a remote plant site and is transported to the prepared road bed. The mixture is then dumped in place, spread by a paving machine, and finally compacted in place by heavy steam rollers.
Plant-mix production consumes large quantities of materials, typically as much as 6000 tons per day, thus possibly depleting any given single aggregate source, such as a rock quarry, sand pit, or other source of mineral deposits. Aggregate materials of pavements of several miles or more generally are gathered from several sources and accordingly nonuniform pavements result from source variations. Furthermore during the initiation of a production run, several miles of pavement typically are laid before the materials engineer has acquired sufficient knowledge of the aggregate qualities to establish consistent physical properties in the pavement. In many cases, consistent qualities may never be achieved either because the production job is completed before the material engineer has had the opportunity to correct for source variations, or the nature and quality of the aggregate changes too rapidly for the materials testing engineer to perform required tests and determine appropriate corrective action at the mixing plant. In many cases, the aggregate source intentionally is changed to shorten the distance between the aggregate source and the mixing plant whereupon attempts are then made to bring the new mixture within job specifications.
At the mixing plant, the amount of asphalt cement injected into the mixing bin can be reasonably accurately controlled as a percentage by weight of the total mix. Effective asphalt cement governs the amount of air voids in the compacted mixture and varies as a function of their shape, absorption characteristics, and sizes. Attempts are made to regulate size by controlling gradation over a multitude of screen sizes, generally 8 to 10, ranging from coarse aggregates passing the 1" screen down to mineral filler passing the No. 200 screen. Gradation however is almost impossible to control the job mix formula at the mixing plant and at the lay-down site due to degradation and segregation of aggregates. Plant control in accordance with changes in absorption characteristics, aggregate shapes, or other natural qualities are at best based upon trial-and-error techniques.
No method previously exists for quickly measuring changes in the natural aggregate qualities at the mixing plant site to effect corrective action at the mixing plant during operation. Further, no method previously exists for quickly and accurately determining the necessary corrective action for effecting such proper control of the production plant at the plant site. Thus uniform asphalt pavements cannot be produced.
Frictional qualities also are difficult to achieve for dense graded mixes. An open graded overlay is now used for friction overlays. Frictional qualities are provided by the sharpe edges of 1/2" to 1" coarse aggregate particles protruding upwardly of the riding surface. The asphalt cement in the present open graded frictional overlay is permeable to water and air penetration causing the cement to age prematurely resulting in deterioration of the pavement structure. The underlying surface of the friction overlay also deteriorates for the same reasons. One problem associated with attempts to lay dense graded friction courses is that the coarse rock is pushed down into the underlying asphalt pavement upon load application on the riding surface and thus frictional qualities disappear. Thus to reduce the effects of displacement and reorientation of aggregate particles, the present friction overlay is laid very open graded.
The design of optimum asphaltic mixture today is at best a trial-and-error procedure. Good mixes generally result from knowledge of aggregates, experience, and luck. Many limitations in achieving optimum pavement properties exist, even with the expert design engineer using known aggregates. With some aggregates, present techniques cannot be used to determine a mix design that meets job specifications, such as stability, flexibility, density, or voidage.
In an effort to produce uniform pavements, firstly mix design is performed by trial-and-error methods to determine what aggregate blendings asphalt cement combination will produce certain predesignated pavement qualities such as flexibility, stability, and air voidage; and secondly, crushing, mixing, and laydown operations are controlled to produce an asphalt pavement having the desired physical properties. The object is to produce a pavement so that the load is transmitted to the subgrade only through the rock while the mortar of fine aggegate and cement fill the intersticies between the rock. The aggregates are proportioned according to a predesignated job mix formula which comprises prescribed proportions of different sized aggregates generally ranging from a maximum size of one and one-half inch in diameter to a minimum aggregate size that passes the No. 200 sieve.
Two methods now in general use for mix design and quality testing are the Marshall and the Heevm methods. According to these methods, test specimens are prepared by compacting trial mixes within the limitations of predesignated job mix formula tolerances, and then tested in an effort to determine which blend possesses desired physical properties, such as flexibility, flowability, density, stability, etc. This procedure consumes as much time as two to three weeks. It must be repeated when the natural qualities of the aggregates change during the production operation or when different degrees of segregation and degradation of aggregates occur during stockpiling, transporting and handling.
An asphalt pavement containing a large amount of air voids is an "open graded mix". It is vulnerable to deterioration due to water seepage throughout the pavement. Water weakens the binding effect of the asphalt cement by stripping the cement from the surface of the aggregates. An open graded mix also is subject to aeration which shortens the pavement life due to oxidation of the asphalt cement. Aeration decreases the viscosity of the cement, causing it to become brittle and ultimate breakup of the pavement. Where prolonged road life is not important, an open graded mix, within certain limits, however is used to produce a high friction riding surface. Road pliability having resilient characteristics is sacrificed in open graded mixes.
The nature of the coarse aggregates determine, among other things, the load bearing and riding characteristics of the pavement. Specific qualities of concern include hardness, shape, porosity, and other surface qualities. Coarse aggregate may consists of crushed rock, volcanic rock, hydraulically tumbled stoned, or other large mineral deposits.
A pavement having a small amount of air voids is known as a "dense graded mix". This mixture has a larger quantity of the fine aggregate/cement mortar. Present dense graded mixtures have poor stability and deform under load forces as the coarse aggregates do not properly transmit loads to the subgrade. Where load conditions permit, pavements comprising dense graded mixes are used when high pliability is desired.
A maximum "bulking point" is defined herein as the proportion (percentage by weight of dry coarse aggregate with respect to total weight of dry aggregate in mixture) at which each coarse aggregate particle touches one another while the mortar of fine aggregate and asphalt cement fill all interstitices between the coarse aggregate. The gradation at which the bulking point occurs essentially depends on the shape of the coarse aggregate. For example, the available volume of space between contiguous hydraulically tumbled stone is different than the available volume between contiguous crushed rock, and thus a mixture including one type of coarse aggregate may have a different bulking point than a mixture of another type of coarse aggregate. A mix at or above the bulking point mixture generally is unacceptable because it lacks, among other things, sufficient flexibility, and resistence to air and water penetration. Some presently specified job mix formulas completely ignore bulking point limitations in the specification.
A "balance point" is defined herein as the gradation at which each coarse aggregate particle are in close proximity to one another while the mortar of fine aggregate, asphalt cement, and desired dispersed air voids fill all interstitices between the coarse aggregate. There should be just enough asphalt cement in the mixture to cover the surfaces of all aggregates in the mixture, and just enough air voids to allow for proper expansion and contraction of the pavement under the climatic conditions without penetration of excessive water and air. Thus the air voids are the separating factor between dense and open graded mixes below the bulking point. Excessive asphalt cement reduces stability and too little asphalt cement generally reduces flexibility. The voidage control is difficult to achieve and maintain during the mixing operation because of myriad variables, some of which have been previously indicated, and accordingly, uniform pavements cannot be produced by present techniques.
At least one procedure presently in use for designing a mixture which has the desired quantity of air voids is the manipulation of the job mix formula of discrete sized aggregates over the entire gradation scale. Specifically, Fuller Maximum Density Curves and the Federal Highway Administration 0.45 Power Gradation Chart currently are in use. Their use is described in "Mix Design Methods for Asphaltic Concrete and other Hot-Mix Type" published by The Asphalt Institute, manual series No. 2, fourth edition, March, 1974. The basis for the FHA 0.45 power gradation chart is described in detail in volume 31, pages 176 through 207 of the "Proceedings of the Association of Asphalt Paving Technologist", Jan. 29, 1962. The theory of controlling voidage is based on the principle that a gradation deviating from a maximum density curve will contain increased air voids. Thus some obscure relationship between job mix formula and voidage is developed for job control. It is rarely successful particularly in view of segregation and degradation of aggregates while being handled and transported to and from the crushing plant, stock piles, mixing plant, and laydown site. Degradation between the crushing plant and the mixing plant alone may amount to as much as 30%.
Another method for computing voids in the mineral aggregate is disclosed in volume 34, pages 574 through 594 of the "Proceedings of the Association of Asphalt Paving Technologist". The computations are based upon successive correlations of voids in discrete ranges of the gradation spectrum. Page 577 of the treatise illustrates 8 gradation ranges between mineral filler and 3/8 to 3/4 inch aggregate. By summing the voidage contained in each aggregate group, and considering the correlation factors of aggregate voidage, a sum total is obtained which closely approximates the final aggregate voidage. No clue however is given to how one might achieve this ideal combination of aggregate mixture at the mixing plant nor is any consideration given to variation in effective asphalt cement.
In any job, the use of local indigenous materials is desired, if at all possible, because of expensive transportation cost in transporting aggregates across the country. Certain jobs however do not lend themselves to the use of such indigenous materials because of widely varying changes in natural aggregate qualities that render mixing plant control almost impossible. In that case, materials are imported from a distant source.